1. Field of the Invention
The present invention relates a semiconductor device, and particularly to a semiconductor device such as a power MOSFET.
2. Description of Related Art
In a semiconductor device, such as a power MOSFET (Power metal oxide semiconductor field effect transistor), a gate resistance may be connected to a gate electrode in order to protect a thin gate oxide film. A Zener diode may be connected between the gate electrode and the source electrode to protect the gate oxide film.
For example, Japanese Unexamined Patent Application Publication No. 2002-118258 discloses a trench gate MOSFET. In the MOSFET disclosed in Japanese Unexamined Patent Application Publication No. 2002-118258, a drain region made of an N− type epitaxial layer is provided over an N+ semiconductor substrate, a P type channel layer is provided over the drain region, and a trench is provided in the P type channel layer. Further, a gate oxide film is provided over the P type channel layer, and a Zener diode is provided over the gate oxide film. The Zener diode is formed by alternately ion implanting an N type impurity and a P type impurity into a polysilicon layer. One end of the Zener diode is connected to a gate electrode, and the other end is connected to a source electrode. That is, the Zener diode is connected between the gate electrode and the source electrode. In the Zener diode, an N type semiconductor region doped with an N type impurity and a P type semiconductor region doped with a P type impurity are arranged alternately in a concentric manner. This allows an overvoltage applied between the gate electrode and the source electrode to escape, thereby avoiding to destroy the gate oxide film.
Meanwhile, the Zener diode connected between the gate electrode and the source electrode is often placed below the gate electrode for layout reasons. The configuration of the Zener diode is restricted by a voltage guaranteed value between the gate electrode and the source electrode (VGSS guaranteed value), a diameter of a gate wire, and a characteristic (boundary length) of a diode.
On the other hand, in the field of automotive semiconductors, the size of an electronic circuit to be mounted is increasing along with the progress of the electronic control. Therefore, there is not enough space to mount the electronic circuit inside the vehicle and the electronic devices need to be mounted inside the engine room. Thus, the automotive semiconductors are required for reliability of wider range of temperature. For example, the automotive semiconductors are required for reliability under high temperature of 175 degrees Celsius. Therefore, in the field of automotive power MOSFET, a high temperature reverse bias test is carried out at 175 degrees Celsius and 168 hours or more, for example.
However, when performing the high temperature reverse bias test to a power MOSFET disclosed in Japanese Unexamined Patent Application Publication No. 2002-118258, there was a problem that a leakage current between gate and source increases.
Accordingly, Japanese Unexamined Patent Application Publication No. 2002-043574 discloses a trench gate MOSFET in which two Zener diodes are connected in series between a gate electrode and a source electrode. This trench gate MOSFET attempts to reduce the leakage current by providing an N type semiconductor region between the two Zener diodes.